Looking for a fantasy novel w/dragon on cover

I'm looking for a book, the only problem is I can only remember the cover art, and some of the story itself, not the author or title. I don't mind saying that it's driving me up the wall.
So the cover of the book has a flying dragon that's purple and looks like it's made outta crystal or diamond and it's flying over a castle or tower surrounded by green stuff, grass or trees.
The bits I remember have golden dragons that shift into human form, a grey drake (kinda demi dragon) in human form with one hand, who has his hand regrown. They end up fighting off an invasion of the crystal/gem dragons. The main character becomes Diamond, god of the good aligned dragons. Also the main bad guy is a crystal who creates a giant body for himself. The main character also is said to have fox like features or is called fox-something.
I think it was also part of a series.
I read the book in the later half of the 90's... I think.

I found the book, many thanks to Gilliam who pointed me at novels published through TSR.
The book was called Dragonmage of Mystara, book 3 of the Dragonlord Chronicles based in the D&D world of Mystara.